1. Field of the Invention
The present invention relates to a wire saw for cutting wafers from a workpiece, in particular for cutting semiconductor wafers from rod-shaped or block-shaped semiconductor material. The invention also relates to a sawing method in which a wire saw according to the invention is used.
2. The Prior Art
Wire saws for cutting wafers from a workpiece are already known. The essential components of these wire saws are a machine frame, a feed device, and a roller system combined to form a sawing head and comprising a plurality of wire guide rollers which are rotatably mounted and of which at east one is driven. The actual sawing tool is a web of moving wire segments which are aligned to be parallel and which are stretched between the wire guide rollers. The wire segments may belong to a single finite wire which is coiled around the roller system and which is unwound from a feed roller onto a take-up roller. U.S. Pat. No. 4,655,191, on the other hand, discloses a wire saw in which a multiplicity of finite wires is provided and each wire segment of the wire web is assigned to one of these wires. EP-522,542 A1 also discloses a wire saw in which a multiplicity of endless wire loops run round the roller system. During the sawing operation, the feed device effects a mutually oppositely directed relative movement of the wire segments, which run in guide grooves perpendicularly to the axes of rotation of the wire guide rollers, and of the workpiece. As a consequence of this feed movement and of the supplying of a material-eroding sawing aid which is also described as "slurry", the wire segments work through the workpiece with the formation of parallel sawing gaps. DE-3,942,671 A1 discloses both the feed devices which feed the workpiece toward the stationary wire web and those which feed the cutting head of the wire saw toward the stationary workpiece. In principle, a plurality of wire gratings of a wire saw can also be used for simultaneously machining a plurality of workpieces. Published Specification EP-433,956 A1 describes, for example, a wire saw with which two workpieces can be sawn simultaneously.
The production of semiconductor wafers from rod-shaped or blocked-shaped semiconductor material, for example from monocrystalline rods, imposes particularly high requirements on the wire saw. As a rule, the sawing method has the objective that every semiconductor wafer sawn has side surfaces which are as flat as possible and are situated parallel to one another. The so-called "warp" of the wafers is a known measure of the deviation of the actual wafer shape from the desired ideal shape. The "warp" can, as a rule, be no more than a few .mu.m Every wire segment of the wire web should therefore ideally be situated in the flat cutting plane intended for it at every instant in the sawing operation. The cutting planes intended are imaginary planes which are situated parallel to one another and which pass through the workpiece and between which the semiconductor wafers to be cut from the workpiece are situated. Every movement of the wire segments transversely to the intended cutting planes has the result that the shape (geometry) of the semiconductor wafers produced will deviate from the target requirement.
The causes which result in an incorrect position of the wire segments in relation to the intended cutting planes include, in particular, cutting forces which occur during sawing and which deflect the wire segments from the desired position. Also there are axial displacements of the wire guide rollers due to thermal expansion as a consequence of the heating of the drive of the wire guide rollers. In addition, there are the bearings of the wire guide rollers and the wire guide rollers themselves and play within the bearings as a result of the constant loading of the bearings over time periods of a few hours for one sawing operation. The feed device is also a source of error if the feed guide is not manufactured with adequate precision or if cutting forces cause the feed frame to stretch. In the two cases mentioned last, a movement of the workpiece occurs which is transverse to the intended cutting planes and which also has de facto the result that the wire segments are situated outside the intended cutting planes.
In order to prevent movements of the workpieces or of the wire segments which are transverse to the intended cutting planes, the machine frame and the supporting parts of the feed device and of the sawing head are, as a rule, constructed from heavy and mechanically durable parts, temperature-control devices are also being provided for their thermal stabilization.